The need to remain competitive in cost and performance in the production of semiconductor devices has caused a continuous increase in device density of integrated circuits. To accomplish higher integration and miniaturization in a semiconductor integrated circuit, miniaturization of a circuit pattern formed on a semiconductor wafer must also be accomplished.
A basic photolithographic process, which is the standard technique utilized to manufacture semiconductor wafers, includes projecting a patterned light source onto a layer of photo-reactive or radiation sensitive material, such as a layer of photoresist, which is then followed by a development step. The minimum feature size that a projection system can print is given approximately by:CD=k1·λ/NA where CD is the minimum feature size or the critical dimension; k1 is a coefficient that encapsulates process-related factors; λ is the wavelength of light used; and NA is the numerical aperture of the lens, as seen from the semiconductor wafer. In pursuit of reduced critical dimensions, increasing the numerical aperture NA of exposure systems and/or increasing the coefficient k1 facilitates reduced feature size CD. However, reducing the minimum feature size CD through such efforts has its technological challenges. Thus, exploiting light sources that provide reduced wavelengths, such as in the extreme ultraviolet (EUV) range, is another approach that is currently receiving much attention.
Extreme ultraviolet lithography (EUVL) with an imaging wavelength at 13.5 nm is an attractive solution for realizing the reduction in critical dimensions in semiconductors. However, obstacles have been encountered that have kept EUVL from becoming useful in high volume manufacturing.
One obstacle is critical dimension roughness in the developed layer of photoresist. When defining lines with small widths or critical dimensions (CD) and close pitch distances, variations that occur in patterning such features become problematic due to the small size and closeness of features. One such variation, known as “line width roughness” (LWR), is a deviation in the width or CD of a line feature due to a variation in peak-to-valley amplitude of a non-uniform line edge along its length.
Two basic types of defects are associated with LWR. The first defect is a failure to remove all of the photoresist material from the surface of the underlying layer, leaving a “photoresist scum” that can cause fabrication failures. The second defect, called “footing”, is the retention of photoresist material along the edges of the exposed photoresist, decreasing the width of the openings in the pattern. Attempts at using EUV radiation to image a layer of photoresist that is directly applied to a substrate have encountered increased LWR, (i.e., both scumming and footing). In an effort to improve this critical dimension roughness, one approach has been to utilize an organic under-layer, which has provided some improvement. At certain EM wavelengths, such as 365 nm, 248 nm and 193 nm, under-layers, such as anti-reflective coatings (ARC), are necessary to reduce optical reflections. However, for EUVL, the reflections between a layer of photoresist and a substrate are sufficiently small that an ARC is not necessary.
Referring to FIGS. 1A-1C, a film stack 100 of the prior art is provided having a substrate 101 coated with an organic under-layer 102 and an imaged layer of photoresist 103, wherein exposed regions 104 are shown. Where the layer of photoresist 103 is a positive tone photoresist comprising a photoacid generator, exposed regions 104 are rendered soluble to positive tone developing chemistry, such as aqueous tetramethylammonium hydroxide (TMAH), upon performing a post-exposure bake. As shown in FIG. 1B, exposure of layer of photoresist 103 to a developing chemistry removes exposed regions 104 to provide openings 105. Ideally, the exposed regions 104 are developed evenly and completely to provide a bottom surface 109 of opening 105 with a width 108 and having a low LWR, as shown in FIG. 1C. Additionally, side walls 106 of opening 105 should have low line edge roughness (LER). To complete the image transfer to substrate 101, additional processing, such as an etching step, is subsequently required.
Referring to FIGS. 2A-2C, a film stack 200 of the prior art is provided having a substrate 201 coated with a layer of photoresist 203 without an organic under-layer. After exposing the layer of photoresist 203 to patterned EUV radiation, exposed regions 204 are formed. When the layer of photoresist 203 is a positive tone photoresist that comprises a photoacid generator, exposed regions 204 are rendered soluble to positive tone developing chemistry, such as aqueous TMAH, upon performing a post-exposure bake. As shown in FIG. 2B, exposure of layer of photoresist 203 to a positive-tone developing chemistry removes exposed regions 204 to provide openings 205. While side walls 206 of opening 205 do not show a substantial change in LER, the bottom surface 209 of opening 205 shows an increased LWR and incomplete development of exposed region 204 (i.e., scumming). An organic under-layer may contain a mobile acidic species that is capable of migrating into the layer of photoresist 203 and/or the under-layer may inhibit or minimize acid migration from the layer of photoresist 203. In either case, the increased LWR is presumably attributed to the absence of the organic under-layer.
Thus according to the prior art, an organic under-layer improves LWR for EUVL processing, but the organic under-layer increases processing times and adds to the cost of EUVL. As such, it would be advantageous to develop new methods of patterning substrates that overcome the issues of the prior art.